Electrosurgery is a widely used surgical procedure for treating tissue abnormalities. For example, it is known to use radio frequency (RF) energy to treat or ablate cancerous lesions in the liver, kidney, lungs and other tissues. RF ablation occurs as a result of a high frequency alternating current (AC) flowing from the tip of an electrode through the surrounding tissue. Ionic agitation is produced in the tissue around the electrode tip as the ions attempt to follow the change in direction of the alternating current. This ionic agitation creates frictional heating and necrosis of the tissue around the electrode. Such procedures may be performed through an open abdominal incision or via laparoscopy, which is performed through multiple, small skin incisions, and can also be conducted percutaneously.
Electrosurgical devices that can be used for tissue ablation using RF energy generally fall into one of two categories, monopolar devices and bipolar devices. Monopolar electrosurgical devices typically include an electrosurgical probe having a first or “active” electrode extending from one end. The electrosurgical probe is electrically coupled to an electrosurgical generator, such as a RF generator, which provides a high frequency electrical current. During an operation, a second or “return” electrode, having a much larger surface area than the active electrode, is positioned in contact with the skin of the patient. The surgeon may then bring the active electrode in close proximity to the tissue and activate a switch, causing electrical current to flow from the distal portion of the active electrode and through tissue to the larger return electrode.
Bipolar electrosurgical devices do not use a return electrode. Instead, bipolar devices include a second electrode that is positioned adjacent to the first electrode. Both electrodes are attached to an electrosurgical probe. As with monopolar devices, the bipolar electrosurgical probe is electrically coupled to an electrosurgical generator. When the generator is activated, electrical current flows from the end of the first electrode through intervening tissue to the end of the adjacent second electrode.
Referring to FIGS. 1 and 2, one known bipolar electrosurgical probe 10 includes a shaft or cannula 20 that includes a proximal shaft, cannula or conductive element 22 (generally referred to as a proximal cannula) and a distal shaft portion, cannula or conductive element 24 (generally referred to as a distal cannula). An insulative member 26 separates and electrically isolates the proximal and distal cannulas 22 and 24. The outer surface of the shaft 20 includes an insulative coating 28. The proximal cannula 22 is electrically isolated from the distal array 34, and the distal cannula 24 is electrically isolated from the proximal array 32.
Referring to FIGS. 1 and 3, individual electrodes of the proximal and distal electrode arrays 32 and 34 are initially retained inside the shaft 20. During use, the distal end of the shaft 20 is inserted into diseased tissue, and individual electrodes 36 of the proximal electrode array 32 are deployed through ports 42 defined by a proximal cannula 22, and individual electrodes 38 of the distal electrode array 34 are deployed through ports 44 defined by the distal cannula 24. Deployment is performed using one or more reciprocating shafts or other components, e.g., as described in U.S. Publication No. 2005/0080409, the contents of which are incorporated herein by reference.
In the illustrated device, the deployed electrode arrays 32 and 34 face each other. This arrangement is referred to as a symmetric or mirrored arrangement since a balanced current density exists between the two electrode arrays 32 and 34. More particularly, referring to FIG. 3, electrical current flows between an active array (+) 34 and a return array (−) 32. Ablation regions or lesions 52 and 54 (generally referred to as an ablation lesion) initially form around the tips of the individual electrodes 36 and 38.
With continued application of current, ablation lesions 52 and 54 symmetrically grow inwardly and eventually meet in a middle region between the electrode arrays 32 and 34 to ablate the middle portion of diseased tissue. Symmetrically configured probes that operate in this manner are otherwise described as probes that perform ablation in an “outside-in” manner.
Referring to FIG. 4, it is also known to use bipolar electrosurgical probes 60 that are asymmetric in that the proximal and distal arrays 32 and 34 face the same direction, and there is an unbalanced current density and unbalanced formation of ablation lesions between the electrode arrays 32 and 34. More particularly, referring to FIG. 5, electrical current (represented as arrows) flows between an active electrode array (+) 34 and a return electrode array (−) 32.
Referring to FIG. 6, an ablation lesion 72 initially forms around an arcuate surface of electrodes 36 of the proximal electrode array 32, and other, smaller ablation lesions 74 form around the distal tips of individual electrodes 38 of the distal electrode array 34. As shown in FIG. 6, the resulting ablation is unbalanced and biased around the proximal electrode array 32 as a result of low current density along the shaft 20 (generally illustrated in FIG. 8), and the larger surface area of the electrode array 32 compared to the tips of the electrodes 38 of the distal electrode array 34. Thus, ablation around the distal electrode array 34 lags behind ablation around the proximal electrode array 32. Referring to FIG. 7, as additional current is applied to the probe 60, over time, the ablation lesion 72 and the smaller ablation lesions 74 grow and eventually fill in the space between the electrode arrays 32 and 34 until the ablation lesions 72 and 74 meet in a middle region.
Thus, similar to the ablation probe 10 shown in FIGS. 1 and 3 having electrode arrays 32 and 34 that face the same direction, probes 60 shown in FIGS. 4-7 having arrays 32 and 34 that face different directions may also initially form ablation lesions around the outer electrode arrays 32 and 34, which grow and migrate inwardly toward the center or a middle region between the electrode arrays 32 and 34.
Uneven ablation patterns may result in an “hour glass” shaped lesion due to ablation migrating inwardly from the outer electrodes and towards the middle region. The middle region of diseased tissue, which is often the bulk of the tissue to be treated, may be only partially ablated or not ablated at all. This may be common if the procedure is interrupted.
Other known probes include electrode arrays that face opposite directions (symmetrical configuration) and include an additional electrode array to boost the ablation in the middle region. Such probes may improve upon hour glass ablation patterns, but they also use additional electrode arrays and involve more complicated structural configurations in order to connect, insulate and deploy the array components.
Probes having electrode arrays facing the same direction (asymmetrical configuration) also exhibit “hour glass” ablation patterns. Further, such probes typically involve longer ablation times for the middle region of diseased tissue to be ablated. Accordingly, it would be desirable to have electrosurgical probes that are able to form larger and more complete ablation lesions in less time. Further, it would be desirable to reduce or eliminate “hour glass” shaped lesions.